A class-D amplifier, sometimes known as a switching amplifier, is an electronic amplifier in which all transistors operate as binary switches. They are either fully on or fully off CLASS-D amplifiers employ rail-to-rail output switching, where ideally their output transistors virtually always carry either zero current or zero voltage. Thus, their power dissipation is minimal, and they provide high efficiency over a wide range of power levels. Their advantageous high efficiency has propelled their use in various audio applications, from cell phones to flat screen televisions and home theater receivers. Class-D audio power amplifiers are more efficient than class-AB audio power amplifiers. Because of their greater efficiency, class-D amplifiers require smaller power supplies and eliminate heat sinks, significantly reducing overall system costs, size, and weight.
Class D audio power amplifiers convert audio signals into high-frequency pulses that switch the output in accordance with the audio input signal. Some class D amplifier use pulse width modulators (PWM) to generate a series of conditioning pulses that vary in width with the audio signal's amplitude. The varying-width pulses switch the power-output transistors at a fixed frequency. Other class D amplifiers may rely upon other types of pulse modulators. The following discussion will mainly refer to pulse width modulators, but those skilled in the art will recognize that class D amplifiers may be configured with other types of modulators.
FIG. 1A shows a simplified schematic diagram illustrating a conventional class-D amplifier 100. As shown, amplifier 100 includes two comparators 101 and 102. Amplifier 100 also includes an oscillator 103, which outputs a clock signal OSC_CLK and a triangular wave signal VREF. The waveforms for clock signal OSC_CLK and triangular wave signal VREF are shown in FIG. 1A as inserts below the block diagram for oscillator 103. Differential input audio signals INP and INM are input to comparators 101 and 102, where input signals INP and INM are compared with triangular wave signal VREF to generate PWM signals 106 and 107. PWM signals 106 and 107 are coupled to the gates of transistors M1, M2, M3, and M4, respectively. Differential output signals OUTM and OUTP of the class D amplifier are respectively provided at terminals also labeled OUTM and OUTP. As shown in FIG. 1, output signals OUTM and OUTP are connected to a speaker load 110, which is represented schematically by an inductor L1 and a resistor R1.
Traditional class D amplifiers have differential outputs (OUTP and OUTM) wherein each output is complementary and has a swing range from ground Vss to Vdd. FIG. 1B is a waveform diagram illustrating the modulation of signals in the class-D amplifier of FIG. 1. As shown in the top diagram FIG. 1B, differential input signals, e.g., audio signals INM and INP, are compared with a triangular reference waveform VREF by two comparators as described above in connection with FIG. 1. The output signals of the comparators are pulse signals at a fixed frequency whose pulse width is proportional to the input signal. Two PWM signals are shown in FIG. 1B as OUTP and OUTM.
Filter-less Class-D Audio amplifiers, such as amplifier 100 of FIG. 1, employ a modulation scheme often referred to as BD-modulation. In this modulation scheme the output drivers, connected in bridge tied load configuration, switch the positive and negative side of the load to: 1) Vdd & GND; 2) GND & Vdd; 3) Vdd & Vdd; 4) GND & GND, where Vdd is the supply voltage and GND is the supply ground. As a result, the differential voltage across the load has three levels: 1) Vdd; 2) −Vdd; 3) 0. For 0 level audio output voltages the differential voltage across the load will be predominantly zero, allowing filter-less operation through an inductive speaker load.
BD modulating class-D amplifiers are sometimes referred to as “filter-free” because no LC filter is required to improve small signal efficiency. A problem with class-D amplifiers is caused by clipping. Such conditions can occur, for example, when input signals exceed the proper range for the class-D amplifier. Clipping is undesirable for many reason, for example, it can cause signal distortion and cause undue stress on the speaker system. More details on signal clipping are described in the sections below.
From the above, it is clear that an improved method improving class-D amplifiers is highly desirable.